FIELD OF THE INVENTION
The invention covers a steam generator and a steam turbine driving unit containing the generator which can be operated in a more advantageous way than conventional combustion engines, without air contamination, especially if hydrogen is used for fuel, because in this case the carbon dioxide emission accompanying the firing of hydrocarbons is avoided.
The most widely used fuels are hydrocarbons, primarily crude oil derivates. Consequently, oil reserves are expected to be exhausted sometime in the middle of the next century, according to prevailing estimates. Another disadvantage is the carbon dioxide emission, the harmful consequences of which are increasingly grave. With higher firing temperatures, the further drawback of nitrogen oxide emission arises, increasingly jeopardizing life and health.
To reduce the above indicated problems, the use of hydrogen as fuel has been considered for a long time. Hydrogen is available in unlimited quantities, and returns to its former state after firing. Hydrogen is the cleanest fuel, can be produced and used without losses, in a cycle, without emitting any substances than could be harmful to the environment. As the firing temperature can be reduced with suitable control, the production of nitrogen oxides can be reduced to a minimum level.
The general use of hydrogen as fuel requires the construction of various projects to resolve the task related to the production, storage and transportation of large quantities.
However, the use of hydrogen also creates a number of problems. At an environmental temperature, gaseous molecular hydrogen does not penetrate metals immediately. Hydrogen in atomic form is more dangerous. Atomic hydrogen can be generated at a temperature of above 430.degree. F., in the presence of humidity, in case of corrosion and electrolysis, as well as a higher hydrogen pressure. At higher temperatures, the effect of hydrogen consists of a surface or inner decarbonization. The surface decarbonization occurs above 1050.degree. F., and this reaction is caused by water vapor. During the inner decarbonization, hydrogen penetrates into steel at temperatures above 430.degree. F., reducing the iron carbide to metallic iron by means of an internal reaction, creating methane. The products of the reaction gather at grain boundaries and in small crevices, causing the reduction of plasticity. In graver cases, they bring about the growth of local, internal pressures, and consequently the formation of blisters or fractures. (See P. Webb; C. Gupta: metals in hydrogen environment, Chem. Eng. October, 1984). The stress type fractures caused by hydrogen are sometimes mentioned as a delayed rigid fracture caused by hydrogen or an internal hydrogenous rigidity. This used to happen when the metal was exposed for a long period of time to tensile stresses caused by load lower than the yield limit.
At lower temperatures, below 430.degree. C., brittleness or the formation of blisters takes place. The brittleness or rigidity is caused by the hydrogen penetrating into the metal, reducing the plasticity and tensile strength of the metal.
The effect of hydrogen on metals can be reversible or irreversible. In case of an irreversible rigidity, the absorption of hydrogen is accompanied by a damage arising in the metal structure, which remains even when the last traces of the hydrogen itself have left the metal. One example is copper, in which hydrogen, having diffused into the metal at a temperature of above 300.degree. C., enters into reaction with the inclusions, whereby water vapor and metallic copper are produced. The increased pressure associated with high temperature is sufficient for the inclusions to widen themselves, making the texture of the metal porous and deteriorating its strength. The reversible transformations can be reversed by driving out the hydrogen, and the original value of plasticity can be regained.
Hydrogen as a fuel has very good characteristics: its combustion temperature and heating value are high, it is able to burn at a low concentration, without producing any smoke. These features allow it to be burned in an internal space (oxidation), as the thermal energy can be used with a high efficiency.
The burning of hydrogen in a combustion chamber is advantageous compared with natural gas. It is known that the radiation coefficient of the flame (in case the flame does not contain solid particles) is directly related to the triatomic gases, i.e. in this case with carbon dioxide and water.
In its combustion product (containing exclusively water vapor, having a higher radiation than that of carbon dioxide), the hydrogen gas has a higher quantity of triatomic gases, therefore its radiation ability is higher than that of natural gas. The radiation of a hydrogen flame is higher than that of natural gas, and its outer parts are hotter, therefore its radiation is good.
With respect to heat transfer characteristics, the adiabatic temperature (2100.degree. C.) of hydrogen is higher than that of natural gas (1950.degree. C.). The combustion of hydrogen requires less air (0.80 m.sup.3 /n/kWh) than does natural gas (0.96 m.sup.3 /n/kWh). For a given power and torque, the heat transfer of hydrogen gas is 10% better than that of natural gas.
The hydrogen/air mixture is ignited at a mixture of 4 as well as 75% and has a high inflammation speed. The flame supplied by hydrogen during tests is extremely stable, being calm without artificial stabilization, at any pressure level.
All of this shows that the burning of hydrogen does not represent any difficulty either in an open or a closed combustion space. Its good radiation, heat transfer and stability facilitate energy transformation with high efficiency. Its efficiency can be considerably higher than that of hydrogen burned in a combustion engine, because the high temperature and energy of the chemical reaction accompanying the explosion can only be used with a considerable loss, due to friction, the permanent cooling applied because of over-heating, and the cooling by water injection applied for the reduction of nitrogen oxide development. At the same time, the transformation from the gaseous state into the liquid state (development of water and water vapor) influence the resulting pressure conditions in a disadvantageous way, and the metal parts of the combustion engine are exposed to intensive corrosion by the created water fraction.
It is obvious that--despite the many advantageous characteristics of hydrogen--a number of obstacles have to be overcome to use hydrogen as fuel.
U.S. Pat. No. 4,573,435 (SHELTON) describes a method and apparatus for the production of hydrogen to be used as a diesel engine additive. In this procedure, water is sprayed on a pipe bundle of a heat exchanger delivering the hot exhaust gases. The velocity of spraying is so high that a considerable part of the water disintegrates into hydrogen and oxygen. This gas mixture and the remaining steam are fed into a combustion chamber after being mixed. The approach has the disadvantage that only a small amount of thermic water decomposition can be achieved in this way, and the obtained hydrogen and water vapor damage the combustion engine considerably.
U.S. Pat. No. 4,253,428 (BILLINGS et al.) describes a hydrogen fuel system used together with, or as an alternative to, a hydrocarbon fuel system in a vehicle, containing a combustion unit as well as a unit mixing the introduced hydrogen with air.
This solution also has the disadvantage that the combustion engine is damaged by hydrogen. Also, nitrogen oxide contaminants are produced during combustion at high temperatures, the quantity of which cannot be reduced.
U.S. Pat. No. 4,528,947 (OLIVERA) describes an apparatus functioning with solar oxy-hydrogen where the electrolyzation cell producing hydrogen and oxygen is integrated into the cooling system of a combustion engine. One of the electrodes consists of the engine-house, the other is the cooler. The developing hydrogen is stored by the hydride contained in the electrolyte.
This solution has the disadvantage that the combustion engine is damaged by the hydrogen, penetrating into the metal and causing its destruction. A further disadvantage of this solution is that there are nitrogen oxide contaminants produced during the combustion at a high temperature.
Patent No. EP 0 153 116 (SUTABIRAIZA CO.) describes a method for producing mechanical energy by means of a multi stage utilization of H.sub.2 O plasm. The plasm is obtained through water dissociation and the reactive pressure is maintained by the plasm. The mechanical energy is gained by the explosion of the electrically conducting plasm in a piston engine.
This solution has the disadvantage that the metallic structure of the piston engine is damaged by the hydrogen. A further disadvantage of this solution is that the piston engine is overheated during the combustion of the plasm at a very high temperature, even though appropriate cooling is applied, leading to an intensive wear.
The hydrogen can be stored and transported in the known way, i.e. absorbed in metal, in the form of a metallic hydride, in a hydride container.
The firm of Daimler-Benz performed tests with vehicles provided with the above-mentioned hydride containers. The volume of the hydride accumulator amounted to 65 liters, its mass to 200 kg, thus a distance of 200 km could be covered with an engine having an output of 44 kW. A doubling of the distance to be covered was expected from an improvement of the hydrides. Similar tests have been performed elsewhere, too, and it was found that the traditional combustion engines used in vehicles can be used with hydrogen without any difficulty. Considerable changes were only needed at the carburetor and the ignition units. This way, the former engine structure could be retained and an entirely new engine need not be developed. This approach is of limited utility, however, because only a low efficiency can be achieved if the traditional engines are fueled with hydrogen, and the hydrogen has a damaging effect on the metallic environment, and this effect cannot be eliminated. Therefore, changes are necessary, in the form of a driving unit resisting the damaging effect of hydrogen and having a suitable efficiency, allowing its economic use in an era of limited energy resources.